According to non-patent document 1 (“Applied Physics Letters” 84, page 4041), a thin layer of gallium nitride is grown on a sapphire body, and the gallium nitride layer and a surface of the sapphire body are then etched by H2 annealing. The surface of the sapphire body is etched by the etching to form fine roughness thereon. By re-growing a gallium nitride thin layer on the roughened surface, it is possible to produce a seed crystal substrate including voids. However, the non-patent document 1 does not disclose Na flux method utilizing a gallium nitride seed crystal substrate.
Further, patent document 2 (Japanese Patent Publication No. 2000-327495A) discloses Na flux method of utilizing a seed crystal substrate of gallium nitride. In growing gallium nitride single crystal by Na flux method, by utilizing a substrate including deposited thin layer of gallium nitride (or AlN thin layer), it becomes possible to control the positions of nucleation and to facilitate the growth. However, according to the patent document 2, it is not described to peel GaN template from the thus grown gallium nitride layer.
According to patent document 3 (Japanese Patent Publication No. 2004-247711A), it is formed a gallium nitride layer with a layer including voids and a gallium nitride layer is then grown thereon by flux method so that a substrate and the grown nitride crystal are separated from each other in the vicinity of the voids.
According to patent document 4 (WO 2009-011407), a surface of a sapphire body is etched to produce a substrate including gallium nitride seed crystal with a layer including voids. A thick film of a gallium nitride crystal layer is then grown by flux method. During the subsequent fall of temperature, the sapphire body and gallium nitride are spontaneously peeled off from each other around the voids due to a difference of thermal expansion coefficients of sapphire and gallium nitride, so that a self-standing body of gallium nitride is obtained.